You need a "logdata 0" and "logdata 1" block around the first navigation so it doesn't get captured. The video (and WebPagetest UI) only know how to work with a single step so you need to structure your scripts so that only one step is recorded. The sample here shows what you need to do: http://www.webpagetest.org/docs/Hosted_Scripting

You are seeing the 2 videos essentially interleaved because it doesn't know what to do with the results.